Solubility of lactose

The solubility characteristics of the a- and -isomers are distinctly different. When a-lactose is added in excess to water at 20°C, about 7 g per 100 g water dissolve immediately. Some a-lactose mutarotates to the P anomer to establish the equilibrium ratio 62.7jS 37.3a therefore, the solution becomes unsaturated with respect to a and more a-lactose dissolves. These two processes (mutarotation and solubilization of a-lactose) continue until two criteria are met 7 g a-lactose in solution and a jS/a ratio of 1.6 1.0. Since the P/x ratio at equilibrium is about 1.6 at 20°C, the final solubility is 7 g -1- (1.6 X 7) g = 18.2 g per 100 g water. 

When -lactose is dissolved in water, the initial solubility is 50 g per 100 g water at 20°C. Some )8-lactose mutarotates to a to establish a ratio of 1.6 1. At equilibrium, the solution would contain 30.8 g p and 19.2 g a/100 ml therefore, the solution is supersaturated with a-lactose, some of which crystallizes, upsetting the equilibrium and leading to further mutarotation of p-i cc. These two events, i.e. crystallization of a-lactose and mutarotation of p, continue until the same two criteria are met, i.e. 7 g a-lactose in solution and a p/x ratio of 1.6 1. Again, the final solubility is 18.2g lactose per 100 g water. Since -lactose is much more soluble than a and mutarotation is slow, it is possible to form more highly concentrated solutions by dissolving P- rather than a-lactose. In either case, the final solubility is the same. 

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Whey concentration, both of whole whey and ultrafiltration permeate, is practiced successfully, but the solubility of lactose hmits the practical concentration of whey to about 20 percent total sohds, about a 4x concentration fac tor. (Membranes do not tolerate sohds forming on their surface.) Nanofiltration is used to soften water and clean up streams where complete removal of monovalent ions is either unnecessary or undesirable. Because of the ionic character of most NF membranes, they reject polyvalent ions much more readily than monovalent ions. NF is used to treat salt whey, the whey expressed after NaCl is added to curd. Nanofiltration permits the NaCl to permeate while retaining the other whey components, which may then be blended with ordinaiy whey. NF is also used to deacidify whey produced by the addition of HCl to milk in the production of casein. 

Lactose, the milk sugar, is a reducing disaccharide consisting of glucose and galactose moieties. The estimated annual worldwide availability of lactose as a byproduct from cheese manufacture is several million tons [1,2], but only about 400 000 t/a lactose is processed further from cheese whey [3], Non-processed whey is an environmental problem due to its high biochemical and chemical oxygen demand [2], The use of lactose as such is limited by two main factors relatively low solubility of lactose in most solvents and lactose intolerance in human body [1]. 

The solubility of lactose as a function of temperature is summarized in Figure 2.5. The solubility of a-lactose is more temperature dependent than that of p-lactose and the solubility curves intersect at 93.5°C. A solution at 60°C contains approximately 59 g lactose per 100 g water. Suppose that a 50% solution of lactose ( 30g /J- and 20 g a-) at 60°C is cooled to 15°C. At this temperature, the solution can contain only 7 g a-lactose or a total of 18.2 g per 100 g water at equilibrium. Therefore, lactose will crystallize very slowly out of solution as irregularly sized crystals which may give rise to a sandy, gritty texture. 

Figure 2.5 Solubility of lactose in water (modified from Jenness and Patton, 1959).

As discussed in section 2.2.5, the solubility of lactose is temperature dependent and solutions are capable of being highly supersaturated before spontaneous crystallization occurs and even then, crystallization may be slow. In general, supersolubility at any temperature equals the saturation (solubility) value at a temperature 30°C higher. The insolubility of lactose, coupled with its capacity to form supersaturated solutions, is of considerable practical importance in the manufacture of concentrated milk products. 

In frozen milk products, lactose crystallization causes instability of the casein system. On freezing, supersaturated solutions of lactose are formed e.g. in concentrated milk at -8°C, 25% of the water is unfrozen and contains 80 g lactose per 100 g, whereas the solubility of lactose at — 8°C is only about 7%. During storage at low temperatures, lactose crystallizes slowly as a monohydrate and consequently the amount of free water in the product is reduced. 

The solvent and the presence of salts or sucrose influence the solubility of lactose, as well as the rate of mutarotation. The solubility of lactose increases with increasing concentrations of several calcium salts—chloride, bromide, or nitrate—and exceedingly stable, concen-... 

It has been shown that calcium chloride also markedly increases the solubility of lactose in methanol (Domovs and Freund 1960). From the highly concentrated viscous solutions formed there slowly crystallizes a complex of /3-lactose, calcium chloride, and methanol in a molecular ratio of 1 1 4. On addition of water to the concentrated solution, the complex previously described (a-lactose CaCli 7H20) soon crystallizes. 

Some studies have been made on the effects of other sugars on the solubility of lactose (Nickerson and Moore 1972). At 10 to 18°C, a 14% sucrose solution, comparable to that in ice cream mix, reduces lactose solubility only slightly. However, the data in Table 6.3 show that concentrations of 40 to 70% sucrose reduce the solubility of lactose appreciably—to 40 to 80% of normal. At temperatures near 0°C, the solubility of lactose is reduced by about one-half by saturating the solution with sucrose. 

As mentioned previously, alcohol greatly reduces the solubility of lactose, but the glass or amorphous form dissolves in alcoholic solutions to form supersaturated solutions. This has been used to extract lactose from whey or skim milk powder with methanol or ethanol. A high-grade lactose subsequently crystallizes from the alcoholic solu-... 

Table 6.3. Relative Solubility of Lactose in Sucrose Solutions. ...

Acetone also reduces the solubility of lactose, upon which a procedure to recover lactose from whey is based (Kerkkonen et al. 1963). Acetone is added to concentrated whey (18 to 20% lactose) in amounts sufficient to precipitate some of the impurities. After these are filtered out, the gradual addition of acetone to over 65% allows recovery of 85% of the lactose during a 3.5 hr period. The yield of lactose and rapidity of crystallization are influenced by the rate of acetone addition. 

Herrington, B. L. 1934B. Some physico-chemical properties of lactose. VI. The solubility of lactose in salt solutions the isolation of a compound of lactose and calcium chloride. J. Dairy Sci. 17, 805-814. 

The solubility of lactose is less than that of most other sugars, which may present problems in a number of foods containing lac-... 

Figure 4-16 Solubility of Lactose in Water. Source From E.O. Whittier, Lactose and Its Utilization A Review, J. Dairy Sci., Vol. 27, p. 505, 1944.

The solubility of lactose in water is known over a wide temperature range, 390,402 solubility at 25° being 17.8 g./lOO ml. of solution, and its... 

The product contains 440/260 = 1.69 kg sucrose per kg water and, according to Table 2.2, the solubility of lactose then is close to 200 g per kg water. This implies that about 58 g of lactose will eventually crystallize. Assuming that the number of crystals formed equals the number of seed crystals added, the volume of a seed crystal must be at most 0.3/58 times the volume of a crystal eventually formed. Assuming the crystals to be of the same shape, the maximum diameter of a seed crystal should then be (0.3/58)1/3 x 8 pm = 1.38 pm. 

V should not be determined from that asymptote because considerable error can be introduced. It is worthwhile to remind that V is a limit and certainly not the maximum attainable value of v, since substrate solubility in the reaction medium can be far away from the theoretical saturation value for the enzyme. As an example, the maximum value of v attainable for the hydrolysis of lactose with (3-galactosidase is only 88% of V, since the solubility of lactose at the conditions of reaction is about 250 g (Illanes et al. 2000). If K is determined from that value of V (substrate... 

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